Work in progress involves the determination of the location, regulation, secretion, and assembly of cholera toxin subunits into holotoxin. Prior studies have involved the development of rapid tissue culture methods for the large scale screening of non-toxinogenic and hypo-toxinogenic mutants of Vibrio cholerae. Studies on the in vivo genetic stability of mutant strains using germ free rats are in progress. The results from these studies will be used for the determination of appropriate characteristics to be employed in the construction of V. cholerae strains to be used for live oral immunization against cholera and related enterotoxic enteropathies.